call. our report tonight from nbc's ron mott. >> reporter: today, defense attorney mark o'mara challenged trayvon martin's father about whether he told police the first time he heard the 911 call that it was not his son's voice screaming for help. >> i kind of pushed away -- away from the table and shook my head and said i can't tell. >> reporter: the prosecutor later listened to the tape at the mayor's office 20 times. >> basically what i was listening to. i was listening to my son's last friday for he cry for help, listening to his life being taken, and i was coming -- trying to come to grips that trayvon was here no more. >> reporter: earlier, the lead investigator offered a different reaction to the 911 tape. >> i inquired if that was, in fact, his son yelling for help. >> what was his response? >> he -- he was more of a -- verbal and nonverbal. he looked away and under his breath and said. no. >> reporter: one after another -- >> help! >> so you think he's yelling help? >> yes. >> reporter: five of zimmerman's friends testified that its of his voice. >> definitely georgie.